Method for manufacturing molten galvanized steel sheet

Abstract

The present invention relates to a molten metal plated steel sheet manufacturing method for cooling a molten galvanized layer with high efficiency when manufacturing a molten galvanized steel sheet, and the purpose of the present invention is to provide a method for manufacturing a molten galvanized plating, wherein a molten galvanized steel sheet having an aesthetically pleasing surface without fitting defects, drop mark defects, and linear comb-pattern defects can be stably obtained by cooling a galvanized layer with high efficiency during a molten metal plated steel sheet manufacturing process. This method for manufacturing a molten galvanized steel sheet having excellent surface properties is characterized by comprising the steps in which a molten galvanized layer is formed on the surface of a steel sheet while the steel sheet passes through a galvanizing pot, the thickness of the galvanized layer formed on the surface of the steel sheet is adjusted while the steel sheet passes through a gas wiping device, the steel sheet that has had the thickness of the galvanized layer adjusted undergoes a primary cooling while passing through a bottom cooler, and the galvanized steel sheet that has undergone the primary cooling undergoes a secondary cooling while passing through a cooling chamber, wherein: the primary cooling is performed with cooling air blown from the bottom cooler until right before a galvanizing solution of the galvanized layer attached to the surface of the steel sheet becomes solidified, the amount of air blown being adjusted according to the temperature of the galvanized layer attached to the surface of the steel sheet; and the secondary cooling is performed with ionic air generated from an ionic air generator provided in the cooling chamber and a spray solution sprayed from a solution atomization part, the secondary cooling being performed from the start of the solidification of the galvanizing solution until the end of the solidification, and the cooling chamber cooling the galvanized steel sheet while moving up and down according to the temperature of the galvanized layer attached to the surface of the galvanized steel sheet.

The invention claimed is: 
     
       1. A method of manufacturing a hot-dip galvanized steel sheet having excellent surface quality, in which:
 a hot-dip galvanized layer is formed on a surface of a steel sheet as the steel sheet passes through a galvanizing pot; 
 a thickness of the galvanized layer formed on the surface of the steel sheet is adjusted as the galvanized layer passes through a gas wiping apparatus; 
 the steel sheet whose galvanized layer thickness is adjusted is primarily cooled while passing through a bottom cooler; and 
 the primarily cooled galvanized steel sheet is secondarily cooled while passing through a cooling chamber, 
 wherein the primary cooling is carried out using cooling air supplied from the bottom cooler until immediately before a galvanizing solution in the galvanized layer attached to the surface of the steel sheet solidifies, and a volume of the air is adjusted according to a temperature of the galvanized layer attached to the surface of the steel sheet, and 
 the secondary cooling is carried out from a time point at which the solidification of the galvanizing solution in the galvanized layer begins to a time point at which the solidification ends, by using ionic wind generated by an ionic-wind generator provided in the cooling chamber and a solution sprayed from a solution atomizing unit, and the cooling chamber is moved up or down according to the temperature of the galvanized layer attached to the surface of the steel sheet being galvanized. 
 
     
     
       2. The method of  claim 1 , wherein the adjustment of the volume of the air of the bottom cooler and the up or down movement of the cooling chamber are controlled by a separately provided control unit in accordance with a relative relationship between the temperature of the galvanized layer attached to the surface of the steel sheet and a reference temperature set according to a condition of the galvanized layer, and
 a temperature of the steel sheet entering the cooling chamber is 419° C. or more, and a temperature of the steel sheet exiting the cooling chamber is 418° C. or less. 
 
     
     
       3. The method of  claim 2 , wherein the temperature of the galvanized layer attached to the surface of the steel sheet is measured by optical thermometers set at an emissivity of 0.12 and installed above and below the cooling chamber, and a reference temperature for initiation of the solidification is 340° C., and a reference temperature for termination of the solidification is 380° C. 
     
     
       4. The method of  claim 3 , wherein the volume of the air of the bottom cooler is controlled as follows:
 when T pt -T pb  is 20° C. or less, and at the same time, T pb  and T pt  are greater than Ts, or when T pt -T pb  is greater than 20° C., and at the same time, T pb  is smaller than Tl, and T pt  is between Ts and TI, the volume of the air of the bottom cooler reduced; 
 when T pt -T pb  is 20° C. or less, and at the same time, T pb  is smaller than Ts, and T pt  is greater than Ts, the volume of the air of the bottom cooler is increased; and 
 when T pt -T pb  is greater than 20° C., and at the same time, T pb  is smaller than Tl, and T pt  is greater than Ts, the volume of the air of the bottom cooler is maintained, 
 (wherein, T pt  is a temperature measured by the optical thermometer set at an emissivity of 0.12 and installed above the cooling chamber, T pb  is a temperature measured by the optical thermometer set at an emissivity of 0.12 and installed below the cooling chamber, Tl is a reference temperature for a molten galvanized layer input to the control unit, and Ts is a reference temperature for a solidified galvanized layer input to the control unit). 
 
     
     
       5. The method of  claim 3 , wherein the up or down movement of the cooling chamber is controlled as follows:
 when T pt -T pb  is greater than 20° C., and at the same time, T pb  is smaller than Tl, and T pt  is between Ts and Tl, the position of the cooling chamber is maintained; 
 when T pt -T pb  is 20° C. or less, and at the same time, T pt  and T pb  are greater than Ts, the cooling chamber is moved down, and, after the moving down of the cooling chamber, when T pt -T pb  is 20° C. or less, and at the same time, T pb  is smaller than Ts, and T pt  is greater than Ts, the cooling chamber is moved further down; and 
 when T pt -T pb  is greater than 20° C., and at the same time, T pb  is smaller than Tl, and T pt  is between Ts and Tl, the cooling chamber is moved up, 
 (wherein, T pt  is a temperature measured by the optical thermometer set at an emissivity of 0.12 and installed above the cooling chamber, T pb  is a temperature measured by the optical thermometer set at an emissivity of 0.12 and installed below the cooling chamber, Tl is a reference temperature for a molten galvanized layer input to the control unit, and Ts is a reference temperature for a solidified galvanized layer input to the control unit). 
 
     
     
       6. The method of  claim 2 , wherein a flow velocity of the ionic wind is increased by air supplied frog an air injection device installed at the rear of the ionic-wind generator. 
     
     
       7. The method of  claim 2 , wherein the ionic wind is generated by the ionic-wind generator in which a direct-current high voltage is superposed with a pulse high voltage. 
     
     
       8. The method of  claim 1 , wherein a flow velocity of the ionic wind is increased by air supplied from an air injection device installed at the rear of the ionic-wind generator. 
     
     
       9. The method of  claim 1 , wherein the ionic wind is generated by the ionic-wind generator in which a direct-current high voltage is superposed with a pulse high voltage.